1. Field of the Invention
The present invention relates to a writable optical disk recording apparatus for writing arbitrary data on a writable optical disk by the radiation of a light beam or laser beam.
2. Description of the Prior Art
WORM (write once and read many) type optical disks (hereinafter simply referred to as a writable disk) in which an organic pigment is coated on one surface of a disk the size of which is the same as that of, for example, a compact disc (CD) and arbitrary data is written on one surface by the radiation of a laser beam are now commercially available. According to this optical disk, written data thereon can be reproduced by using ordinary compact disc reproducing apparatus and this optical disk can be utilized to manufacture a small number of compact discs.
Such writable optical disks have a very small difference in sensitivity among organic pigments coated by different manufacturers. Although the difference of sensitivity, of course, falls within an allowable range determined in a specification or the like, depending on a combination of a disk, a recording apparatus and a reproducing apparatus, there is then the risk that the disk cannot be reproduced.
Signals are recorded on a CD and a CDROM (compact disc read-only memory) are by a modulation system generally called an EFM (eight-to-fourteen modulation) system. As these signals, there are utilized signals having lengths of nine kinds from 3T to 11T as shown in FIG. 1A of the accompanying drawings. When an RF (radio frequency) signal reproduced from the disk on which these signals are randomly recorded is observed through an oscilloscope, then eye pattern waveforms shown in FIGS. 2A to 2C of the accompanying drawings are obtained. By examining a characteristic (asymmetry) of the eye pattern waveform, it is possible to understand the output state of a laser beam relative to an organic pigment. FIGS. 2A through 2C show the change of characteristic (asymmetry) of a reproduced signal relative to an output of a laser beam and a sensitivity of organic pigment. FIG. 2A shows the condition such that a sensitivity of organic pigment is low and the output of laser beam is set in a so-called under-power. In this case, a pit formed on the optical disk is small and a degree that a radiated laser beam is irregularly reflected on the pits is small so that the radiated laser beam approaches a mirror level of total reflection. In this case, the characteristic (asymmetry) of the reproduced signal approaches an upper side (mirror level). FIG. 2C shows the condition such that a sensitivity of organic pigment is high and the output of the laser power is set in a so-called over-power. An area of pit formed on the optical disk is increased and a radiated laser beam is reflected irregularly so that the level of the reproduced signal from the reflected laser beam is lowered. Therefore, the characteristic (asymmetry) of the reproduced signal from the reflected laser beam is moved toward the lower side.
When the sensitivity of the organic pigment and the output of the laser beam are set in a proper relationship, as shown in FIG. 2B, the characteristic (asymmetry) of the reproduced signal is located at the center of the signal amplitude. Thus, an eye pattern that is used to detect a signal becomes smaller as the characteristic (asymmetry) of the reproduced signal comes away from the center of the signal amplitude, thereby making it impossible to reproduce the signal.
To solve the above problem, it is proposed to adjust an output power of a recording laser beam by identifying a signal that was written for a trial on a power control area (PCA) provided at one portion of the disk. FIG. 3 of the accompanying drawings schematically shows one surface of a disk D. As shown in FIG. 3, a program recording area Dp is provided on the disk D similarly to the compact disc, a lead-in area Di including a TOC (table of contents) in which playing time data of program or the like is recorded is provided at the inner peripheral side of the program recording area Dp, and a lead-out area Do is provided at the outer peripheral side of the program recording area Dp.
Further, on the inner peripheral side of the lead-in area Di, there are provided a program memory area PMA in which playing time data of programs recorded until the recording of all programs is completed, etc., are recorded and a power control area PCA which is used to adjust the output power of a laser beam by checking a sensitivity of organic pigment coated on the disk D. Before the recording is started, an arbitrary signal is recorded (recorded for test) on the power control area PCA while the output level of the laser beam is sequentially changed. Then, the output level at which the reproduced signal of the signal thus recorded comes to have a predetermined characteristic (asymmetry) is determined and the output power of the laser beam is adjusted to this determined output level.
When the output power of the laser beam is adjusted by writing the arbitrary signal on the power control area PCA for a test, the adjusted output power value of the laser beam is maintained during the recording. Therefore, if a wavelength of a laser diode that emits a laser beam is displaced due to the change of temperature and the like during the recording or a sensitivity of organic pigment becomes different on the inner and outer peripheries of the optical disk, then the above previously-proposed technique becomes useless.
It is customary that an output power of the laser diode is controlled by detecting (i.e., so-called front monitor) the intensity of laser beam. This conventional method, however, cannot eliminate the disadvantages brought about when the sensitivity of the organic pigment coated on the disk surface is changed due to the fluctuation of wavelength of the laser diode or a sensitivity becomes different on the inner and outer peripheries of the optical disk.